A technique for welding materials known as friction stir welding is recognized among the conventional methods of joining substrates. In conventional frictional stir welding, a non-consumable cylindrical tool may have a flat surface (i.e. shoulder) on the working end with a pin in the center. When the tool is plunged into a substrate, or between adjacent substrate pieces, the flat surface rests flush against the surface of the substrate, and the pin extends into the substrate. The tool is rotated and simultaneously translated relative to the substrate. The force of the translation and rotation plasticizes the substrate in the vicinity of the shoulder and pin. The rotation blends the substrate and moves substrate around the pin which permits the pin to move through the substrate. The plasticized substrate material is relocated to the wake of the pin, where it forms a weld bead. Importantly, the substrate material is plasticized, as opposed to melted, during this operation. Melting is specifically avoided, so that properties of the weld, which is made of plasticized and then solidified substrate, remain close to those of the substrate itself.
In a recent variation, filler material is fed through the non-consumable tool. This filler material exits the tool and contacts the substrate, where it is then plasticized and mixed into the weld by the weld tool. Using filler material permits the composition and volume of the weld bead to be adjusted as desired because the filler material is dispersed throughout the entire volume of the weld bead. This is different from friction surfacing, where a consumable material is rotated on a substrate's surface to generate heat via frictional contact. In friction surfacing heat causes plastic deformation which leaves some of the consumable material on the surface of the substrate. This is different than friction stir welding because surface treatment seeks only to treat the substrate's surface while avoiding significant penetration of the rotating material into the substrate. In friction surfacing only the surface of a substrate is built upon. Deep penetration of the consumable material in friction surfacing is not possible and not intended because the consumable is meant to deposit on the surface thereby building it up and (often) imparting different surface composition and properties.
Conventional friction stir welding has disadvantages. For example, once a weld bead of desired length is reached, the tool must either lift out of (i.e. retract from) the substrate, or must run past the edge of the substrate. In the first case, the tool will leave a weld tool crater or cavity (hereafter referred to as a crater) of tool tip geometry in the weld bead as it is extracted from the substrate because it is no longer plasticizing substrate, so there is no plasticized substrate to move into its wake. In the second case, once the leading edge of the tool reaches the edge of the substrate, it is again without material to plasticize and direct toward its wake, and a void may be left in the weld bead at the edge of the substrate. Run off tabs may be used to supply the feed of plasticized material but these tabs are often difficult to incorporate (i.e. fixture and support), especially on edges of geometrically complex parts. Thus there is room for improvement in the art.